NCV53480 Datasheet, PDF (54/60 Pages) ON Semiconductor – Low-Power OOK/FSK/ASK ISM Band Transceiver

NCV53480
Combined RX/TX Match
The RFIN and RFOUT pins can be shorted together using
the matching network given in the TX Matching section to
provide a single RF port without the need for an external
RX/TX switch. In this configuration, the achievable RF
output power will be slightly less due to the increased
capacitance on the output net due to the LNA input. The
RFPWR pin is pulled to ground internally during receive to
provide the necessary DC path for the LNA input to ground.
For applications where it is desirable to have the RFIN and
RFOUT ports separated to facilitate the addition of an
external LNA, PA, or SAW filter network, the RXactive pin
is provided for control of these external devices. The pin as
its name implies is high during receive, and low otherwise.
DAC
RFPWR
The current sink/source capability of the RXactive pin is
limited to $1 mA.
Matching to Small Loop Antenna
In many applications, the radiating antenna will not be a
50 W antenna, but rather a small printed loop antenna. In this
case, the Antenna itself will form the inductor Ls and load
resistance shown as Rrad. Typical values for Ls for a printed
copper loop are between 40nH and 100nH, while Rrad is less
than 1 W. This leads to having a very high Q tank, and hence
a high output impedance seen by the PA at resonance which
can cause damage to the device due to excessive voltage
swings if care is not taken. Increasing the value of Cp will
reduce the impedance transformation ratio and protect the
device from damage.
RFC
LNA
VCO
RFIN
RFOUT
C_s
Ls
C_P
Rrad
RFVSS
Figure 37. Loop Antenna Mat
Matching Tips
While matching the LNA input is straightforward and can
be done quickly with the suggested network and the use of
a network analyzer, matching the PA output is less
straightforward. A good way to optimize the PA match is to
utilize the frequency agility of the NCV53480. A simple
application which sweeps the RF frequency of the
NCV53480 while using a spectrum analyzer and DMM to
measure the output power and supply current draw can
quickly provide a designer with the necessary information
to refine the matching network. The graph below shows such
a sweep performed on a device with the match listed in
Table 16 for 433.92 MHz. Notice that the points of
maximum output power and minimum current draw do not
coincide. The point of minimum current draw will always be
slightly higher in frequency than the peak power point. So,
while this match can deliver approximately the same amount
of power at 400 MHz as it can at 450 MHz, the current
consumption will be 25% higher at 400 Mhz than at
450 MHz. To adjust the frequency of the PA match, start
with the match from the table closest to the desired
frequency, then adjust the values of Ls and Cs using F =
1/sqrt(LC) keeping Ls as large as practical for maximum Q.
Keep in mind that PCB trace will add ~1 nH per mm, so keep
traces short.
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